A large static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This large static magnetic field is referred to as the B0 field.
During an MRI scan, Radio Frequency (RF) pulses generated by one or more transmitter coils cause a called B1 field. Additionally applied gradient fields and the B1 field cause perturbations to the effective local magnetic field. RF signals are then emitted by the nuclear spins and detected by one or more receiver coils. These RF signals are used to construct the MR images. These coils can also be referred to as antennas. Further, the transmitter and receiver coils can also be integrated into one or more transceiver coils that perform both functions. It is understood that the use of the term transceiver coil also refers to systems where separate transmitter and receiver coils are used.
MRI scanners are able to construct images of either slices or volumes. A slice is a thin volume that is only one voxel thick. A voxel is a small volume element over which the MR signal is averaged, and represents the resolution of the MR image. A voxel may also be referred to as a pixel (picture element) herein if a single slice is considered.
Dixon methods of magnetic resonance imaging include a family of techniques for producing separate water and lipid (fat) images. The various Dixon techniques such as, but not limited to, two-point Dixon methods, three-point Dixon methods, and multi-point Dixon methods are collectively referred to herein as Dixon techniques or methods. The terminology to describe the Dixon techniques is well known and has been the subject of many review articles and is present in standard texts on Magnetic Resonance Imaging. For example, the “Handbook of MRI Pulse Sequences” by Bernstein et al., published by Elsevier Academic Press in 2004, contains a review of some Dixon techniques on pages 857 to 887.
The journal article Ma J et al., Magn Reson Med 2008; 60:1250-1255 discloses that large and spatially linear phase errors along the frequency encoding direction may be induced by several common and hard-to-avoid system imperfections such as eddy currents. Such linear phase errors can pose challenges to the phase correction algorithms commonly applied in Dixon processing. This article further discloses a two-step process that first corrects the linear component of the phase errors with a modified Ahn-Cho algorithm (Ahn C B et al., IEEE Trans Med Imaging 1986; 6:32-36) and then corrects the residual phase errors with a previously-developed region growing algorithm (Ma J, Magn Res Med 2004; 52:415-419).
U.S. Pat. No. 4,885,542 A discloses performing at least one extra NMR measurement cycle without imposing any magnetic gradients during readout and recordation of the NMR RF response. Calibration data derived from this extra measurement cycle or cycles can be used for resetting the RF transmitter frequency and/or for phase shifting other conventionally acquired NMR RF response data to compensate for spurious changes in magnetic fields experienced during the NMR data measuring processes.
The journal article Yu H et al., J Magn Reson Imaging 2010; 31:1264-1271 discloses that bipolar data may be subject to asymmetric amplitude modulations due to the receive filter response. This article further discloses correcting for these phase and amplitude errors by collecting a small number of phase-encoded lines with reversed gradient polarities.